Fire Scar Histories

Adapted from: Swetnam, Thomas W., Craig D. Allen and Julio L. Betancourt. Applied historical ecology: Using the past to manage the future. Ecological Applications 9:4 p. 1189.

Repeated surface fires cause a sequence of overlapping wounds. The heat-killed wood tissues extend into the annual rings, which can be dated to the calendar year.

Prior to the 19th century westward expansion and Euro-American settlement in North America, frequent, low intensity surface fires were a dominant ecological process in most pine and mixed-conifer forests. Detailed and long chronologies of surface fires can be assembled by sampling fire scars preserved within the boles of surviving trees. Most trees producing annual growth rings have also developed a physiological capacity to grow new rings over injuries caused by fires. This capacity, plus the enhanced flammability of exposed wood and seeping resin within previously formed wounds, often leads to multiple, overlapping fire scars on individual trees. The calendar years and usually the approximate seasons of past fires can be determined by crossdating the annual rings and by carefully observing the position of fire scars within the dated rings. Compiled fire-scar records from numerous specimens collected throughout forest stands provide nearly complete inventories of fire events over periods of centuries.

Fire-scar histories are limited in a number of important ways. They are generally precluded across large areas; past harvesting and recent, intense fires can alter or remove the accumulated record; the fire-scar record generally decreases in completeness with increasing time before the present because of burning and decay processes; and the spatial resolution of past fire perimeters may be low.

Although these problems constrain our ability to reconstruct or use fire history for some purposes, the tree-ring record has an advantage of high temporal resolution. Seasonal or annual dataing resolution of fire scars enables fire historians to evaluate patterns of fire event synchrony across multiple spatial scales. From evaluation of these patterns it is possible to make reasonable inferences about past fire extent, and human versus natural causes of change. Patterns of change in the fire-scar record are interpretable in the context of climatic variation and changes in land use and forest stand structures (including fuel conditions). Thus, fire histories record the ecological "pulse" of southwestern forests, integrating both natural and cultural histories.

Fire-Southern Oscillation Relations in the Southwestern United States. A close linkage between fire and climate could diminish the importance of local processes in the long-term dynamics of fire-prone ecosystems. The structure and diversity of communities regulated by fire may have nonequilibrial properties associated with variations in global climate. Successful prediction of vegetation change hinges on a better understanding of climatically driven disturbance regimes and the relative contributions of regional versus local processes to community dynamics. Adapted from a journal article by Thomas W. Swetnam and Julio L. Betancourt.

Changed Southwestern Forests: Resource effects and management remedies. Over 150 years of occupancy by northern Europeans has markedly changed vegetative conditions in the Southwest. Less fire due to grazing and fire suppression triggered a shift to forests with very high tree densities, which in turn contributed to destructive forest fires. Options to deal with these changes include prescribed fire, thinning and timber harvest to mimic natural disturbances and conditions. However, there are barriers to implementing these activities on a scale large enough to have a significant benefit. Adapted from a published journal article by Marlin Johnson.

Allen, C. D., Betancourt, J. L. and Swetnam, T. W. 1998. Landscape changes in the southwestern United States: Techniques, long-term data sets and trends. Pp. 71-84 In: Sisk, T. D., editor. Perspectives on the land use history of North America: A context for understanding our changing environment. Biological Science Report USGS/BRD/BSR-1998-0003. U.S. Geological Survey, Biological Resources Division, Reston, VA.

Arno, S. F., Smith, H. Y. and Krebs, M. A. 1997. Old-growth ponderosa pine and western larch stand structures: Influences of pre-1900 fires and fire exclusion. Research Paper INT-RP-495. USDA Forest Service.

Covington, W. W. 1994. Post-settlement changes in natural fire regimes and forest structure: ecological restoration of old-growth ponderosa pine forests. Journal of Sustainable Forestry 2: 153-181.

Dieterich, J. H. 1983. Fire history of southwestern mixed conifer: A case study. Journal of Forest Ecology and Management. 6: 13-31.

Dieterich, J. H. and Swetnam, T. W. 1984. Dendrochronology of a fire-scarred ponderosa pine. Forest Science 30: 238-247.

Dieterich, J. H. and Hibbert, A. R. 1988. Fire History in a small ponderosa pine stand surrounded by chaparral [in central Arizona]. In: Krannes, J. S., editor. Effects of fire management of southwestern natural resources: Proceedings of the Symposium, Tucson, AZ. General Technical Report RM-191. USDA Forest Service, Rocky Mountain Forest and Range Experiment Station, Fort Collins, CO.

Grissino-Mayer, H. and Swetnam, T. W. 1997. Multi-century history of wildfire in the ponderosa pine forests of El Malpais. Pp. 163-172 In: Maybery, K., editor. Natural history of El Malpais National Monument. New Mexico Bureau of Mines and Mineral Resources, Socorro, NM.

Harrington, M. G. and Sackett, S. S. 1992. Past and present fire effects on southwestern ponderosa pine old growth. Pp. 44–50 In: Proceedings of a workshop; Old-growth forests of the Southwest and Rocky Mountain regions. Report RM-213. USDA Forest Service, Rocky Mountain Forest and Range Experiment Station, Fort Collins, CO.

Swetnam, T. W. and Baisan, C. H. 1996. Historical fire regime patterns in the Southwestern United States since AD 1700. Pp. 11-32 In: Allen, C. D., editor. Proceedings of the Second La Mesa Fire Symposium. General Technical Report RM-GTR-286. USDA Forest Service, Los Alamos, NM.

Swetnam, T. W., Allen, C. D. and Betancourt, J. L. 1999. Applied historical ecology: Using the past to manage the future. Ecological Applications 9:4 1189.

Touchan, R., Allen, C. D. and Swetnam, T. W. 1996. Fire history and climatic pattens in ponderosa pine and mixed-conifer forests of the Jemez Mountains, northern New Mexico. Pp. 179-195 In: Allen, C. D., editor. Fire effects in southwestern forests: Proceedings of the Second La Mesa Fire Symposium. General Technical Report RM-286. USDA Forest Service, Fort Collins, CO.